Physicians monitor various physiological parameters in their patients and use the results of such monitoring as an important tool to evaluate the patients' health. The monitoring of cardiovascular function is particularly valuable and is performed on a very widespread basis. Accurate measurement of blood pressure and other physiological signals allow for careful diagnosis of medical problems. Monitoring cardiovascular functions, such as blood pressure, can allow a physician to diagnose conditions such as hypertension (increased blood pressure) which may result from processes such as aging or disease.
The heart functions as a pump which moves blood through the circulatory system by a regulated sequence of contractions. The heart ejects blood into the aorta. The blood then flows through the arteries, arterioles, and capillaries to the tissues where the blood delivers oxygen and other nutrients and removes carbon dioxide and other waste products from the tissues. The blood returns to the heart and the lungs where carbon dioxide is expelled from the body and oxygen is again transported into the body. The human body regulates blood pressure throughout the circulatory system to facilitate efficient delivery of blood to the tissues.
Blood pressure does not remain constant but fluctuates during the pumping cycle of the heart. The maximum blood pressure in each cycle is called the systolic blood pressure ("SBP"). SBP occurs as the heart discharges blood into the aorta and the aorta distends to its maximum with the large volume. The minimum blood pressure in each cycle is called the diastolic blood pressure ("DBP"). DBP occurs at the end of the heart's pumping cycle just before the heart begins another contraction. DBP occurs when the aorta has drained of most of the blood from the previous cycle. The mean arterial pressure ("MAP") is the average of the blood pressure throughout a complete cycle. Mean blood pressure is lower further away from the heart in the circulatory system than it is closer to the heart. Mean blood pressure is also subject to hydrostatic pressure variations. Mean blood pressure tends to be reduced at locations above the heart. Mean blood pressure tends to be greater when measured at locations lower than the heart.
Blood pressure also fluctuates with a wide variety of other factors including activity level, pain, temperature, pharmaceutical agents, stress, and recent smoking or food intake. Studies have shown that the measured blood pressure in a single person can vary considerably over time and in different environments. Results of some of these studies are described in the following references: Watson, R. D. S., et al. "Variation in cuff blood pressure in untreated outpatients with mild hypertension--implications for initiating antihypertensive treatment" Journal of Hypertension, Vol. 5, No. 2, pp. 207-211 1987; James, G. D., et al. "The reproducibility of average ambulatory, home, and clinic pressures" Hypertension, Vol. 11, No. 6, Part 1, pp. 545-549, 1988; and, White, W. B., et al. "Average daily blood pressure, not office blood pressure, determines cardiac function in patients with hypertension" Journal of American Medical Association, Vol. 261, No. 6, pp. 873-877, 1989. Blood pressure is also subject to long-term and more permanent changes due to lifestyle, disease and age.
A complication in taking blood pressure measurements accurately is that many patients suffer from a condition known as "white coat hypertension". White coat hypertension is a false hypertension that is normally caused by stress and/or anxiety resulting from the presence of a physician or nurse. Studies of white coat hypertension have shown that up to 21% of untreated borderline hypertensive patients had white coat hypertension. (Pickering, T. G., et al. "How common is white coat hypertension?" Journal of American Medical Association, Vol. 259, No. 2, pp. 225-228, 1988 and Staessen et al., "Antihypertensive treatment based on conventional or ambulatory blood pressure measurement" Journal of American Medical Association, Vol. 278, No. 13, pp. 1065-1072, 1997.)
White coat hypertension may be reduced with familiarity of the patient with the physician, environment, and/or the technology. For example, it has been shown that the blood pressure readings of patients taken by a physician in a clinical environment on two different days two weeks apart tend to drop with time (James et al., The reproducibility of average ambulatory, home and clinical pressures, Hypertension, Vol. 11, No. 6, Part 1, pp.545-549, 1999).
Various methods are already available for measuring blood pressure. For example, blood pressure may be measured directly in the aorta or other blood vessel. This may be done, for example, by inserting into the blood vessel a probe, such as a needle or catheter which bears, or is attached to, a pressure transducer. The transducer accurately measures the actual pressure of the blood within the blood vessel. Nikolic, U.S. Pat. No. 5,758,652 provides an example of an invasive device capable of making direct blood pressure measurements. The Nikolic device utilizes averaging for smoothing out or removal of respiratory-induced artifacts in an intra-arterial blood pressure signal. Although it is ideal to have directly measured blood pressure values for diagnostic purposes, procedures for directly measuring blood pressure are invasive and are normally restricted to critical care environments such as operating rooms.
Indirect or non-invasive techniques for measuring blood pressure include the traditional method of auscultation in which a blood pressure cuff is inflated to occlude the arteries in the limb (normally the upper arm) and then deflated. During deflation, the physician uses a stethoscope to listen to the Korotkoff sounds (K-sounds) in the blood vessels distal to the blood pressure cuff. The systolic and diastolic pressures are associated with identifiable K-sounds and the cuff pressures at these points are normally measured with a mercury sphygmomanometer. A disadvantage of the auscultation method is that it must be done by a skilled person, such as a physician or nurse.
Another disadvantage of the auscultation method is that it is usually performed by a professional who has limited time. Often only a single measurement is performed. A single measurement may be inaccurate. The American Heart Association recommends that two or more measurements be averaged to produce an accurate determination of blood pressure. Research studies of blood pressure also normally collect multiple readings from patients, and use averaging of the readings in data analysis to limit the effects of fluctuations in pressure and increase accuracy. The measurements should be separated by one to two minutes to allow for release of blood trapped in the veins of the extremity, a wait period that also allows adequate adaptation of the patient to the physician, the environment, and the technology. Making two measurements multiplies the professional time required to obtain a blood pressure measurement.
Barker, U.S. Pat. No. 5,201,320 entitled BLOOD PRESSURE MEASURING DEVICE describes a device designed to address the errors associated with the variability in single blood pressure measurements. The device allows a physician to take two good measurements of blood pressure and to obtain the average of the results automatically. This device requires the physician to use a stethoscope to listen to the K-sounds during automatic deflation of a blood pressure cuff and manually determine the SBP and DBP for each measurement. This device remains subject to the high level of variability and errors associated with white coat hypertension because it requires the presence of a physician or nurse.
The inventors have also conducted unpublished studies which have found that when a series of blood pressure measurements is taken on the same patient the first blood pressure measurement in the series tends to be higher than the values of the other readings. This fact is not generally recognized in the literature and may be explained as results of physical/mental stress experienced by the patients immediately before or during the first blood pressure measurement. The Barker device includes the first blood pressure reading in its average and is therefore less accurate than would be desired.
The prior art includes various devices which automatically take blood pressure measurements by the auscultation method. Such devices typically provide a pressure sensor to measure cuff pressure in place of a mercury manometer, and electronic sound transducers to detect the K-sounds in place of a stethoscope and the trained ear of a physician. Such a device is described in Hutcheson et al., European patent publication No. EP 0-203-181.
Some other automated non-invasive blood pressure measurement systems use an oscillometric technique. In such systems, a blood pressure cuff is inflated to occlude the arteries in the limb (normally the upper arm), and then deflated. During deflation, small oscillations in the cuff pressure are produced by fluctuations in blood pressure in blood vessels underlying the cuff. Pressure transducers accurately measure both the cuff pressure and the small oscillations. Signal processing is then used to determine the SBP, DBP, and MAP from the amplitudes of the oscillations. An oscillometric blood pressure measurement device is described, for example, in Ramsay III U.S. Pat. No. 4,360,029 entitled AUTOMATIC MEAN BLOOD PRESSURE READING DEVICE. This device averages successive pulse oscillations as a noise reduction technique to reduce artifacts and improve pulse pressure measurement accuracy to yield a single MAP reading. More recent prior art devices that implement the oscillometric method are currently used in medicine to provide more accurate measurement of SBP, DBP, and MAP by implementing more advanced signal processing and artifact reduction techniques. Examples of such devices are described in Walloch, U.S. Pat. No. 5,505,206 entitled METHOD AND APPARATUS FOR EXCLUDING ARTIFACTS FROM AUTOMATIC BLOOD PRESSURE MEASUREMENTS and Hersh et al., U.S. Pat. No. 5,590,662 entitled DETECTION OF OSCILLOMETRIC BLOOD PRESSURE COMPLEXES USING CORRELATION. Devices which use oscillometric methods are also subject to the unreliability of a single blood pressure determination. The above-noted AHA recommendations apply to any non-invasive blood pressure methods including manual and automatic auscultation and oscillometry.
Japanese Patent publication No. 63-311929 A discloses an apparatus adapted to acquire and memorize multiple blood pressure measurement values, calculate the average of the values, and display the results selectively by pressing a push button. The device improves measurement accuracy by averaging multiple measurements. However, the device needs to be operated multiple times when multiple measurements are required. When the device is operated by healthcare personnel in a physician's office, errors associated with "white coat hypertension" may still occur.
There exist devices for monitoring blood pressure over time. Such devices typically use one of the above methods to take repetitive measurements of blood pressure and/or other physiological parameters. Such devices do not provide accurate, reliable, single blood pressure readouts for diagnosis of hypertension. The cycle times for automatic repetitive measurements may be fixed by the developer, programmable by the user, or even random as described in Hutcheson et al. European Patent No. EP 0 203 181 B1. Some monitoring devices designed for long-term collection of physiological data require the physician or user to manually initiate each measurement, as described in Yamaguchi, U.S. Pat. No. 4,747,412 entitled ELECTRONIC SPHYGMOMANOMETER WITH GRAPHICAL OUTPUT.
These automatic long term monitoring devices normally contain memory for storage of data, which can be displayed after a period of data collection and/or transferred to other devices for statistical analysis. Monitoring devices used in critical care environments normally have alarms that will detect if physiological parameters move into dangerous levels and then notify the physician. Prior art monitoring devices, such as the devices described in Titus, U.S. Pat. No. 4,404,974 entitled METHOD AND APPARATUS FOR MONITORING AND DISPLAYING HEART RATE AND BLOOD PRESSURE PRODUCT INFORMATION and Oka et al., U.S. Pat. No. 5,836,887 entitled PHYSICAL INFORMATION MONITOR SYSTEM HAVING MEANS FOR DETERMINING REFERENCE RANGE FOR ABNORMALITY DETERMINATION, BASED ON MOVING AVERAGE OF PREVIOUSLY OBTAINED VALUES typically utilize repetitive measurements and signal processing methods such as smoothing or running average filtering to produce trends in the physiological signals and to accurately detect potentially harmful changes in the signals during long-term monitoring. Such devices are not functionally suited for use in primary care facilities such as doctors' offices.
In summary, prior art devices that make a single blood pressure measurement are subject to errors associated with white coat hypertension and the natural variability in normal blood pressure regulation. Errors in blood pressure measurement may lead to misdiagnosis of significant health issues such as hypertension. Misdiagnosis of hypertension may lead to increased risk to the patient through misclassification and improper prescription of treatment and to increased liability of the physician. None of the prior art devices described above address the problem that the first measurement in a series of measurements tends to be unreliable.
There is a need for devices which can be used in primary care facilities for accurately measuring blood pressure and/or other physiological parameters. There is a particular need for such devices which can minimize the effects of white coat hypertension and the unreliability of first measurements.